期望的类型参数，找到的结构

Question

我遇到了一个问题，我将其简化为以下代码：

trait Logger {}

struct DefaultLogger;

impl Logger for DefaultLogger {}

struct A<L> where L: Logger {
    logger: Box<L>,
}

impl<L> A<L> where L: Logger {
    fn new() -> Self {
        let logger = DefaultLogger;

        A {
            logger: Box::new(logger),
                          // ^^^^^^ Here is the problem
        }
    }
}

fn main() {
    let a = A::new();
}

产生以下错误：

error[E0308]: mismatched types
  --> src/main.rs:16:30
   |
16 |             logger: Box::new(logger),
   |                              ^^^^^^ expected type parameter, found struct `DefaultLogger`
   |
   = note: expected type `L`
              found type `DefaultLogger`

当我在普通函数（如 main）中构造特征 A 时，就像我期望的那样。示例：

trait Logger {}

struct DefaultLogger;

impl Logger for DefaultLogger {}

struct A<L> where L: Logger {
    logger: Box<L>,
}

fn main() {
        let logger = DefaultLogger;

        let _a = A {
            logger: Box::new(logger),
        };
}

Answer 1

问题出在这里：

impl<L> A<L> where L: Logger {
    fn new() -> Self {
        let logger = DefaultLogger;

        A {
            logger: Box::new(logger),
        }
    }
}

您想 return A<L> 但您 return A<DefaultLogger> 无法确定 L == DefaultLogger.

要解决此问题，您可以提供专门化，为 DefaultLogger:

创建方法 A::new

impl A<DefaultLogger> {
    fn new() -> Self {
        let logger = DefaultLogger;

        A {
            logger: Box::new(logger),
        }
    }
}

期望的类型参数，找到的结构

expected type parameter, found struct

traits

rust